Nether mobs are not spawning in about a 16 block radius sphere around 0, 64, 0.

I've encountered this in two worlds. The seed to one of which is -3177630428171215745. After hitting this a second time I spent some time checking the area they won't spawn and found it to be a perfect sphere.

Since I can't imagine the use of such a dead zone I figured I would report it as a bug.